Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. While significant advances have been achieved with iron, nickel and cobalt-base superalloys, the high-temperature capabilities of these alloys alone are often inadequate for components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, thermal barrier coatings must have low thermal conductivity and adhere well to the component surface. Various ceramic materials have been employed as the TBC, particularly yttria (Y2O3) stabilized zirconia (ZrO2), commonly known as YSZ. This material is widely employed in the art because it can be readily deposited by plasma spray and vapor deposition techniques. In addition, YSZ has good erosion and impact resistance. An example of the latter is electron beam physical vapor deposition (EB-PVD), which produces a thermal barrier coating having a columnar grain structure that is able to expand with its underlying substrate without causing damaging stresses that lead to spallation, and therefore exhibits enhanced strain tolerance. The component is supported in proximity to an ingot(s) of the ceramic coating material (e.g., YSZ in a vacuum), and an electron beam is projected onto the ingot(s) so as to melt the surface of the ingot and produce a vapor of the coating material that deposits onto the component. Such EB-PVD deposited TBCs are generally deposited to a thickness in the range of about 0.005 inch to about 0.010 inch. Adhesion of the TBC to the component is often further enhanced by the presence of a metallic bond coat, such as a diffusion aluminide or an oxidation-resistant alloy such as MCrAlX, where M is iron, cobalt and/or nickel, and where X is yttria and/or another rare earth oxide.
However, the application of a TBC by the EB-PVD process is expensive and time consuming due to the thickness of the coating. Also, maintenance of the EB-PVD apparatus is performed as a function of operation of the apparatus, so fewer parts having a thick coating can be processed in the period of time between maintenance operations. In addition, the thickness of the coating increases the load on the coated part in a high acceleration G-load environment, particularly for TBC coated blades in a high pressure turbine. One of the properties of the TBC that determines the required thickness of the TBC is the thermal conductivity of the TBC, since a coating with lower thermal conductivity does not have to be as thick as a coating with higher thermal conductivity in order to obtain the same thermal protection for the substrate. Developments in the past have led to TBCs with lower thermal conductivity simply by changing the manner in which the TBC is applied using EB-PVD.
One such method is set forth in U.S. Pat. No. 6,620,465 ('465) to Rigney et al. and assigned to the assignee of the present invention. The '465 patent is directed to a method of improving the thermal conductivity of the TBC resulting from an EB-PVD by moving the article to be coated further from the ingot or source of ceramic material.
In view of the above, there is considerable motivation to further reduce the thickness of the TBC through the use of materials that are lower in thermal conductivity. However, limitations of the prior art are often the result of the relatively narrow range of acceptable and readily available materials. Accordingly, new materials for use in the EB-PVD process are continuously being sought for depositing coatings, and particularly ceramic coatings such as TBCs.
What is needed is a new type of material for use in the EB-PVD process that has lower thermal conductivity, better erosion resistance, and/or better impact resistance than presently available TBC materials and is processible for use in TBC materials. In particular, a material is needed that has a lower thermal conductivity, and at least comparable erosion resistance, and/or impact resistance as YSZ.